The Chemical The Chemical Context of LifeContext of LifeChapter 2Chapter 2

MatterMatter Matter consists of chemical elements in pure Matter consists of chemical elements in pure form and in combinations called compounds; form and in combinations called compounds; living organisms are made of matter.living organisms are made of matter.

MatterMatter -- Anything that takes up space and has -- Anything that takes up space and has mass.mass.

ElementElement -- A substance that cannot be broken -- A substance that cannot be broken down into other substances by chemical down into other substances by chemical reactions; all matter made of elements.reactions; all matter made of elements. Life requires about 25 chemical elementsLife requires about 25 chemical elements 96% of living matter is composed of C, O, H, 96% of living matter is composed of C, O, H,

N.N. Most of remaining 4% is P, S, Ca, K.Most of remaining 4% is P, S, Ca, K.

Trace elementTrace element -- required by organisms in -- required by organisms in extremely small quantities: Cu, Fe, I, etc.extremely small quantities: Cu, Fe, I, etc.

Matter cont.Matter cont. CompoundCompound -- Pure substances made of two or -- Pure substances made of two or

more elements combined in a fixed ratio.more elements combined in a fixed ratio. Have characterisitics different than the elements Have characterisitics different than the elements

that make them up (emergent property).that make them up (emergent property). Na and Cl have very different properties from Na and Cl have very different properties from

NaCl.NaCl.

Difference between mass and weight:Difference between mass and weight: MassMass -- measure of the amount of matter an object -- measure of the amount of matter an object

contains; constant.contains; constant. Weight -- Weight -- measure of how strongly an object is measure of how strongly an object is

pulled by earth's gravity; varies.pulled by earth's gravity; varies.

Nutrient DeficienciesNutrient Deficiencies

Atomic structure determines the Atomic structure determines the behavior of an elementbehavior of an element

AtomAtom -- Smallest possible unit of matter that -- Smallest possible unit of matter that retains the physical and chemical properties retains the physical and chemical properties of its element.of its element.

Subatomic ParticlesSubatomic Particles 1. 1. Neutrons Neutrons (no charge/neutral; found in (no charge/neutral; found in

nucleus; ~ 1 amu).nucleus; ~ 1 amu). 2. 2. Protons Protons (+1 charge; found in nucleus; ~ 1 (+1 charge; found in nucleus; ~ 1

amu).amu). 3. 3. Electrons Electrons (-1 charge; electron cloud; (-1 charge; electron cloud;

1/2000 amu).1/2000 amu). One amu approx equal to 1.7 x 10One amu approx equal to 1.7 x 10-24-24 g. g.

Atomic Number and Atomic WeightAtomic Number and Atomic Weight Atomic numberAtomic number = Number of protons in an = Number of protons in an

atom of a particular element.atom of a particular element. All atoms of an element have the same atomic All atoms of an element have the same atomic

number.number. In a neutral atom, # protons = # electrons. In a neutral atom, # protons = # electrons.

Mass numberMass number -- Number of protons and -- Number of protons and neutrons in an atom; not the same as an neutrons in an atom; not the same as an element's element's atomic weight.atomic weight.

ExamplesExamples 2323Mg Mass number ?? Atomic number ?? Mg Mass number ?? Atomic number ?? 1212

23 1223 12 # of protons ?? # of electrons ?? # of neutrons ??# of protons ?? # of electrons ?? # of neutrons ?? 12 12 1112 12 11

1414C Mass number ?? Atomic number ??C Mass number ?? Atomic number ?? 66 14 614 6 # of protons ?? # of electrons ?? # of neutrons ??# of protons ?? # of electrons ?? # of neutrons ?? 6 6 86 6 8

IsotopesIsotopes

IsotopesIsotopes -- Atoms of an element that have the -- Atoms of an element that have the same atomic number but different mass number; same atomic number but different mass number; different number of neutrons.different number of neutrons.

Half-lifeHalf-life -- Time for 50% of radioactive atoms in a -- Time for 50% of radioactive atoms in a sample to decay. sample to decay.

Biological applications of radioactive isotopes Biological applications of radioactive isotopes include:include:

1. Dating geological strata and fossils.1. Dating geological strata and fossils. Radioactive decay is at a fixed rate; by Radioactive decay is at a fixed rate; by

comparing the ratio of radioactive and stable comparing the ratio of radioactive and stable isotope, age can be estimated. in a fossil with isotope, age can be estimated. in a fossil with thethe

Ratio of Carbon-14 to Carbon-12 is used to Ratio of Carbon-14 to Carbon-12 is used to date fossils less than 50,000 years old.date fossils less than 50,000 years old.

Isotopes cont.Isotopes cont. 2.2. Radioactive tracersRadioactive tracers

Chemicals labelled with radioactive isotopes Chemicals labelled with radioactive isotopes are used to trace the steps of a biochemical are used to trace the steps of a biochemical reaction or to determine the location of a reaction or to determine the location of a particular substance within an organism.particular substance within an organism.

Isotopes of P, N and H were used to Isotopes of P, N and H were used to determine DNA structure.determine DNA structure.

Used to diagnose disease.Used to diagnose disease. 3. Treatment of cancer3. Treatment of cancer

Can be hazardous to cells.Can be hazardous to cells.

Energy LevelsEnergy Levels Electrons are directly involved in chemical reactions.Electrons are directly involved in chemical reactions. They have They have potential energy potential energy because of their position because of their position

relative to the positively charged nucleus.relative to the positively charged nucleus. There is a natural tendency for matter to move to the There is a natural tendency for matter to move to the

lowest state of potential energy.lowest state of potential energy. Different fixed potential energy states for electrons Different fixed potential energy states for electrons

are called are called energy levels energy levels or or electron shells.electron shells. Electrons with lowest potential energy are in energy Electrons with lowest potential energy are in energy

levels closest to the nucleus.levels closest to the nucleus. Electrons with greater energy are in energy levels Electrons with greater energy are in energy levels

further from nucleus.further from nucleus. Electrons may move from one energy level to Electrons may move from one energy level to

another. another.

Electron Configuration and Electron Configuration and Chemical PropertiesChemical Properties Electron configurationElectron configuration -- Distribution of electrons -- Distribution of electrons

in an atom's electron shells; determines its in an atom's electron shells; determines its chemical behavior. chemical behavior.

Chemical properties of an atom depend upon the Chemical properties of an atom depend upon the number number of valence electrons of valence electrons (electrons in the (electrons in the outermost energy level. outermost energy level.

Octet ruleOctet rule -- A valence shell is complete when it -- A valence shell is complete when it contains 8 electrons (except H and He).contains 8 electrons (except H and He).

An atom with an incomplete valence shell is An atom with an incomplete valence shell is chemically reactive (tends to form chemical bonds chemically reactive (tends to form chemical bonds until it has 8 electrons to fill the valence shell).until it has 8 electrons to fill the valence shell).

Atoms with the same number of valence electrons Atoms with the same number of valence electrons show similar chemical behavior.show similar chemical behavior.

Bonding in MoleculesBonding in Molecules Chemical bondsChemical bonds -- Attractions that hold molecules together. -- Attractions that hold molecules together. MoleculesMolecules --Two or more atoms held together by chemical --Two or more atoms held together by chemical

bonds. bonds. Covalent bondCovalent bond -- formed between atoms by -- formed between atoms by sharingsharing a pair a pair

of valence electrons; common in organic compounds.of valence electrons; common in organic compounds. Single covalent bondSingle covalent bond -- Bond between atoms formed by -- Bond between atoms formed by

sharing a single pair of valence electrons.sharing a single pair of valence electrons. Double bondDouble bond -- share two pairs of valence electrons. -- share two pairs of valence electrons. Triple bond Triple bond -- share three pairs of valence electrons.-- share three pairs of valence electrons.

CompoundCompound = A pure substance composed of two or more = A pure substance composed of two or more elements combined in a fixed ratio.elements combined in a fixed ratio.

For example: water (HFor example: water (H22O), methane (CHO), methane (CH44).).

Nonpolar Covalent Bonds Electronegativity -- Atom's ability to attract and hold

electrons. • The more electronegative an atom, the more

strongly it attracts shared electrons. • Scale determined by Linus Pauling: O = 3.5; N = 3.0; S and C = 2.5; P and H = 2.1. Nonpolar bond -- Covalent bond formed by an equal

sharing of electrons between atoms. • Occurs when electronegativity of both atoms is

about the same. • Molecules made of one element usually have

nonpolar covalent bonds (H2 and O2).

Polar Covalent BondsPolar Covalent Bonds Polar bond -- Covalent bond formed by an

unequal sharing of electrons between atoms. • Occurs when the atoms involved have

different electronegativities. • In water, electrons spend more time around

the oxygen than the hydrogens. This causes the oxygen atom to have a slight negative charge and the hydrogens to have a slight positive charge.

Ionic BondsIonic Bonds IonIon -- Charged atom or molecule. -- Charged atom or molecule. Anion --Anion -- An atom that has gained one or more An atom that has gained one or more

electrons from another atom; negatively charged.electrons from another atom; negatively charged. CationCation -- An atom that has lost one or more -- An atom that has lost one or more

electrons; positively charged.electrons; positively charged. Ionic bondIonic bond -- Bond formed by the electrostatic -- Bond formed by the electrostatic

attraction after the complete transfer of an electron attraction after the complete transfer of an electron from a donor atom to an acceptor.from a donor atom to an acceptor.

Strong bonds in crystals, but fragile bonds in water.Strong bonds in crystals, but fragile bonds in water. Ionic compounds are called salts (e.g. NaCl or table Ionic compounds are called salts (e.g. NaCl or table

salt).salt).

Biologically important weak bondsBiologically important weak bonds

Include: Hydrogen bonds; Ionic Include: Hydrogen bonds; Ionic bonds in aqueous solutions; Van bonds in aqueous solutions; Van der Waals forces.der Waals forces.

Hydrogen bondHydrogen bond -- Bond formed -- Bond formed by the charge attraction when a by the charge attraction when a hydrogen atom covalently hydrogen atom covalently bonded to one electronegative bonded to one electronegative atom is attracted to another atom is attracted to another electronegative atom.electronegative atom.

Van der WaalsVan der Waals -- charge -- charge attraction between oppositely attraction between oppositely charged portions of polar charged portions of polar molecules.molecules.